The regulation of gene expression by transcription factors is a fundamental aspect of the physiology of all cells, whether prokaryotic or eukaryotic. In eukaryotic organisms, for instance, a variety of transcription factors govern cell growth, differentiation, and death. The appropriate spatial and temporal expression of specific transcription factors governs development. As examples, transcription factors such as Myc and E2F control progression through the cell cycle; homeodomain, paired box, and forkhead transcription factors, among others, are involved in embryonic development; p53 is involved with tumor suppression and cell death; steroid hormone receptors, such as sex hormone, glucocorticoid, mineralocorticoid, and thyroid hormone receptors have pleiotrophic effects on various aspects of physiology.
The aberrant expression of transcription factors can lead to abnormal development and various disease states. The inappropriate expression of proto-oncogenes such as c-Myc through chromosomal translocation can lead to cancers such as Burkitt's lymphoma. The formation of a PML-RARa fusion protein has been shown to be responsible for acute promyelocytic leukemia. Loss of p53 expression results in increased susceptibility to various cancers. The inappropriate expression or loss of expression of heart specific transcription factors such as Tbx1, Tbx5, NRx2.5, Gata4, Sal4, and Eya4 have been shown to result in congenital heart defects.
Improved methods for regulating gene expression by modulating transcription factor function would result in more optimal treatment of many diseases.
One disease which might be approached by modulating transcription factor function is acquired immune deficiency syndrome (AIDS). Human immunodeficiency virus (HIV) has been identified as the etiological agent responsible for AIDS, a fatal disease characterized by destruction of the immune system and the inability to fight off life threatening opportunistic infections. Recent statistics indicate that as many as 33 million people worldwide are infected with the virus. In addition to the large number of individuals already infected, the virus continues to spread. Estimates from 1998 point to close to 6 million new infections in that year alone. In the same year there were approximately 2.5 million deaths associated with HIV and AIDS.
HIV is a member of the class of viruses known as retroviruses. The retroviral genome is composed of RNA, which is converted to DNA by reverse transcription. This retroviral DNA is then stably integrated into a host cell's chromosome and, employing the replication machinery of the host cells, produces new retroviral particles and advances the infection to other cells. HIV appears to have a particular affinity for the human T-4 lymphocyte cell, which plays a vital role in the body's immune system. HIV infection of these white blood cells depletes this white cell population. Eventually, the immune system is rendered inoperative and ineffective against various opportunistic diseases such as, among others, pneumocystic carini pneumonia, Kaposi's sarcoma, and cancer of the lymph system.
There are currently a number of antiviral drugs available to combat the infection. These drugs can be divided into four classes based on the viral protein they target and their mode of action. In particular, one class of such antiviral drugs are competitive inhibitors of the aspartyl protease expressed by HIV. Other agents are nucleoside reverse transcriptase inhibitors that behave as substrate mimics to halt viral cDNA synthesis. A class of non-nucleoside reverse transcriptase inhibitors inhibit the synthesis of viral cDNA via a non-competitive (or uncompetitive) mechanism. Another class are drugs that block viral fusion. Used alone, these drugs show effectiveness in reducing viral replication. However, the effects are only temporary as the virus readily develops resistance to all known agents.
As indicated above, a number of critical points in the HIV life cycle have been identified as possible targets for antiviral drugs including (1) the initial attachment of the virion to the T-4 lymphocyte or macrophage site; (2) the transcription of viral RNA to viral DNA (reverse transcriptase, RT); and (3) the processing of gag-pol protein by HIV protease. An additional, potentially attractive therapeutic target is transcription of the HIV genome. Transcription of the HIV genome is essential for replication of the virus after integration of viral DNA into a host cell chromosome. However, attempts to target HIV transcription have been hampered, in part, by the fact that transcription of the integrated HIV genome utilizes the host cell transcriptional machinery as well as viral transcription factors. Thus, therapies that attempt to target the transcription of the HIV genome may also interfere with transcription of normal host cell genes. Attempts have been made to target specifically HIV transcription by the generation of dominant negative forms of Tat, a virally encoded transcription factor. However, these dominant forms have been shown to have poor activity at inhibiting HIV transcription and viral replication.
Effective new methods to target underexploited aspects of the HIV lifecycle, such as transcription of the HIV genome would be desirable.